This is a general reply to a few recent comments, you can work out for
yourselves which is applicable where if you've been following this, and
if you haven't then you probably won't care either way.
Yes, measure with ems.
Please apply some "real world" logic to this, differentiation of
Helvetica/Frutiger/Arial/Verdana/Gill/Futura at typical screen
sizes is difficult in the extreme - to campaign for diversity
in this environment seems to challenge the facts.
The x-height analysis in the CSS spec has a high degree of
bogosity. Not only does it attempt to justify itself by
rendering type as image, it also displays such disparate
examples to make the contrasted values meaningless.
The differences in pixel rendered x-heights between any of
the above named sans serifs at text use sizes, as typical
on screen, is negligible.
Text should be rendered to screen by specialised rasterisers
which examine hint data and thus grid-fit the results - this
necessarily homogenises text output for screen.
You don't have to be big to be bold.
-- Clive